The detection and imaging of macromolecules is an essential part of many procedures performed in molecular biology laboratories, such as protein assays, DNA sequencing, and gene mapping. Detection and imaging are generally performed by labeling the molecules of interest with a radioactive species and recording the radioactive emission from the molecules on a film or screen.
Phosphor screens, which have recently been introduced as an alternative to x-ray films as the recording medium, offer the ability to be scanned. This permits the operator to store the recorded data on magnetic or optical media such as computer hard drives, floppy disks and CD ROMs. The recorded information can then be transmitted electronically, and analyzed and manipulated by computer to provide information at a high level of detail.
Phosphor screens are commonly formed by coating a solid substrate with a slurry of phosphor particles dispersed in a liquid resin, then curing the resin to form a solidified layer. The quality of the image which is then stored in the phosphor layer will depend on the gap between the sample and the surface of the layer and on the quality of the surface itself. By limiting the gap, one can maximize the image sharpness by limiting the extent to which the signal emitted by the sample spreads before reaching the phosphor surface. As for the surface, any deviations from a smooth planar surface can affect images recorded from low energy signals since these signals penetrate only a few microns into the phosphor layer. For a .sup.14 C signal, for example, the upper 20 microns of the layer will retain 90% of the signal, while for a .sup.3 H signal, the same proportion will reside within the upper 2 microns. Thus, irregularities in the surface will affect the quality of the image regardless of the depth of the phosphor layer, and the effect of these irregularities is magnified as the gap between the sample and the surface narrows.
A further factor affecting the sensitivity of the screen is the particle size of the phosphors, larger particles having greater sensitivity than smaller particles. The most sensitive particles are those with diameters of 10 microns or greater. Particles of this size and greater tend to produce a surface which is rough and uneven. Also, these particles are easily dislodged from the surface, further adding to the surface unevenness.
In the conventional casting procedure described above, the particles settle randomly as the slurry dries. The surface of the resulting phosphor layer is variable, depending on the particle size and uniformity, the viscosity and consistency of the resin as it affects the settling behavior of the particles, and the manner in which the slurry is dried. Irregularities in the surface contour result from phosphor particles with jagged and pointed edges protruding from the surface.
In addition to obscuring the image, these irregularities make it difficult to apply a protective film over the surface. Protective films protect the surface from moisture, which renders the phosphors susceptible to chemical deterioration, and from physical abrasion, which contaminates the surface with debris that must be removed before the screen can be used to its maximum effect. Particles protruding from the surface tend to pierce the protective film, thus creating passages through which atmospheric moisture can penetrate the film to the underlying phosphor layer. In addition, large protruding particles at the surface can become dislodged and thereby create both large discontinuities in the surface contour and large gaps in the film.
Even when the protective film completely covers the phosphor layer, techniques for applying the film do not compensate for the deformities of the phosphor surface. Depressions in the phosphor surface tend to receive greater amounts of the deposited film than elevated regions, and the difference in film thickness may vary by as much as 10 to 20 microns. For very weak emitters such as .sup.3 H, a variation in film thickness of 1.0 micron can cause a signal attenuation of more than 50%.